Semiconductor device and method of manufacturing the same

ABSTRACT

Provided is a semiconductor device including a first n-type transistor comprising a first work function layer, the first work function layer comprising a first underlying layer; and a second n-type transistor comprising a second work function layer, the second work function layer comprising a second underlying layer. The first and second underlying layers each comprises a metal nitride layer with at least two kinds of metals, and a thickness of the first underlying layer is greater than a thickness of the second underlying layer. A method of manufacturing a gate structure for a semiconductor device is also provided.

CROSS-REFERENCE TO RELATED APPLICATION

This is a continuation application of and claims the priority benefit ofU.S. application Ser. No. 16/884,053, filed on May 27, 2020, nowallowed. The U.S. application Ser. No. 16/884,053 is a continuationapplication of and claims the priority benefit of U.S. application Ser.No. 15/877,391, filed on Jan. 23, 2018, now U.S. Pat. No. 10,707,318,issued on Jul. 7, 2020, and claims the priority benefit of U.S.provisional application Ser. No. 62/586,194, filed on Nov. 15, 2017. Theentirety of each of the above-mentioned patent applications is herebyincorporated by reference herein and made a part of this specification.

BACKGROUND

As the semiconductor industry has progressed into nanometer technologyprocess nodes in pursuit of higher device density, higher performance,and lower costs, challenges from both fabrication and design issues haveresulted in the development of three-dimensional designs, such as a finfield effect transistor (FinFET) and the use of a metal gate structurewith a high-k (dielectric constant) material. The metal gate structureis often manufactured by using gate replacement technologies.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It isnoted that, in accordance with the standard practice in the industry,various features are not drawn to scale. In fact, the dimensions of thevarious features may be arbitrarily increased or reduced for clarity ofdiscussion.

FIG. 1 is a flow chart illustrating a method of manufacturing asemiconductor device in accordance with some embodiments.

FIGS. 2A to 2E are cross-sectional views of a method of manufacturing asemiconductor device in accordance with some embodiments.

FIG. 3A is a perspective view of a FinFET in accordance with someembodiments.

FIG. 3B is a cross-sectional view of the FinFET taken along the lineI-I′ of FIG. 3A.

FIGS. 4A to 4F are enlarged exemplary cross-sectional viewscorresponding to an area of FIG. 3B for various transistors inaccordance with some embodiments.

FIGS. 5A to 5D are exemplary cross-sectional views illustrating variousstages of the sequential fabrication process of a gate structure inaccordance with some embodiments.

FIG. 6 is a flow chart illustrating a method of manufacturing a gatestructure of a semiconductor device in accordance with some embodiments.

DETAILED DESCRIPTION

It is to be understood that the following disclosure provides manydifferent embodiments, or examples, for implementing different featuresof the invention. Specific embodiments or examples of components andarrangements are described below to simplify the present disclosure.These are, of course, merely examples and are not intended to belimiting. For example, dimensions of elements are not limited to thedisclosed range or values, but may depend upon process conditions and/ordesired properties of the device. Moreover, the formation of a firstfeature over or on a second feature in the description that follows mayinclude embodiments in which the first and second features are formed indirect contact, and may also include embodiments in which additionalfeatures may be formed interposing the first and second features, suchthat the first and second features may not be in direct contact. Variousfeatures may be arbitrarily drawn in different scales for simplicity andclarity.

Further, spatially relative terms, such as “beneath,” “below,” “lower,”“above,” “upper” and the like, may be used herein for ease ofdescription to describe one element or feature's relationship to anotherelement(s) or feature(s) as illustrated in the figures. The spatiallyrelative terms are intended to encompass different orientations of thedevice in use or operation in addition to the orientation depicted inthe figures. The device may be otherwise oriented (rotated 90 degrees orat other orientations) and the spatially relative descriptors usedherein may likewise be interpreted accordingly. In addition, the term“made of” may mean either “comprising” or “consisting of.”

The fins may be patterned by any suitable method. For example, the finsmay be patterned using one or more photolithography processes, includingdouble-patterning or multi-patterning processes. Generally,double-patterning or multi-patterning processes combine photolithographyand self-aligned processes, allowing patterns to be created that have,for example, pitches smaller than what is otherwise obtainable using asingle, direct photolithography process. For example, in one embodiment,a sacrificial layer is formed over a substrate and patterned using aphotolithography process. Spacers are formed alongside the patternedsacrificial layer using a self-aligned process. The sacrificial layer isthen removed, and the remaining spacers, or mandrels, may then be usedto pattern the fins.

FIG. 1 is a flow chart illustrating a method of manufacturing asemiconductor device in accordance with some embodiments. FIGS. 2A to 2Eare cross-sectional views of a method of manufacturing a semiconductordevice in accordance with some embodiments.

At Step 10 in FIG. 1 and as shown in FIG. 2A, a substrate 100 isprovided. The substrate 100 includes a region R1 and a region R2. Theregion R1 is used for p-type FinFETs, and the region R2 is used forn-type FinFETs, for example. Although the number of each of the regionsR1 and R2 illustrated in FIG. 2A is one, it should not limit variousembodiments of the present disclosure. In some alternative embodiments,the number of the regions R1 may be three, three regions R1 are used forvarious p-type FinFET with different threshold voltages, such as anultra low threshold voltage (uLVT) p-type FinFET, a low thresholdvoltage (LVT) p-type FinFET, and a standard threshold voltage (SVT)p-type FinFET. Three regions R2 are used for various n-type FinFET withdifferent threshold voltages, such as an uLVT n-type FinFET, a low LVTn-type FinFET, and a SVT n-type FinFET. It will be described in detailin the following paragraphs.

In some embodiments, the substrate 100 includes a bulk substrate, asilicon-on-insulator (SOI) substrate or a germanium-on-insulator (GOI)substrate. In one embodiment, the substrate 100 includes a crystallinesilicon substrate (e.g., wafer). The substrate 100 may include variousdoped regions (such as well regions) depending on design requirements(e.g., p-type substrate or n-type substrate). The doped regions may bedoped with p-type or n-type dopants. For example, the doped regions maybe doped with p-type dopants, such as boron or BF₂ ⁺, n-type dopants,such as phosphorus or arsenic, and/or a combination thereof. In someembodiments, n-type doped regions may be formed in the region R1, andp-type doped regions may be formed in the region R2. In some alternativeembodiments, the substrate 100 may be made of some other suitableelemental semiconductors, such as diamond or germanium, a suitablecompound semiconductor, such as gallium arsenide, silicon carbide,indium arsenide, or indium phosphide, or a suitable alloy semiconductor,such as silicon germanium carbide, gallium arsenic phosphide, or galliumindium phosphide. Also, in some embodiments, the substrate may includeother conductive layers or other semiconductor elements, such astransistors, diodes or the like.

At Step S10 in FIG. 1 and as shown in FIG. 2A, the substrate 100 hassemiconductor fins 101 a and 101 b and isolation structures 201. Thesemiconductor fins 101 a and 101 b are formed of a material the same asor different from a material of the substrate 100. In some embodiments,a depth D1 of the semiconductor fins 101 a and 101 b ranges from 80 nmto 200 nm. The isolation structures 201 include silicon oxide, siliconnitride, silicon oxynitride, a spin-on dielectric material, a low-kdielectric material, or a combination thereof and formed by performing ahigh-density-plasma chemical vapor deposition (HDP-CVD) process, asub-atmospheric CVD (SACVD) process or a spin-on process, for example.

As shown in FIG. 2A, in some embodiments, the semiconductor fins 101 aand 101 b are formed by forming trenches, forming shallow trenchisolation (STI) regions in the trenches, and lowering a top surface ofthe STI regions by performing an etching process to a level lower thanan original top surface of the substrate 100. The remaining portions ofthe STI regions become the isolation structures 201, and the remainingportions of the substrate 100 between the isolation structures 201 thusbecome the semiconductor fins 101 a and 101 b. Top surfaces of theisolation structures 201 are lower than top surfaces of thesemiconductor fins 101 a and 101 b. In other words, top portions 110 ofthe semiconductor fins 101 a and 101 b protrude from the top surfaces201T of the isolation structures 201.

In some other embodiments, the semiconductor fins 101 a and 101 b areformed of a material different from a material of the substrate 100. Thesemiconductor fins 101 a and 101 b may be formed by lowering topportions of the substrate 100 between the adjacent isolation structures201 to form recesses, and re-growing a semiconductor material differentfrom the material of the substrate 100 in the recesses. Top portions ofthe STI regions may then be removed by performing a chemical mechanicalpolish process and an etching process, while bottom portions of the STIregions are not removed. As a result, the remaining portions of STIregions become the isolation structures 201, and top portions of there-grown semiconductor material between the adjacent isolationstructures 201 become the semiconductor fins 101 a and 101 b.

At Step S12 in FIG. 1 and as shown in FIG. 2B, gate stacks 104 a and 104b are formed across the semiconductor fins 101 a and 101 b respectively.In one embodiment, an extending direction X of the gate stacks 104 a and104 b is, for example, perpendicular to an extension direction Y of thesemiconductor fins 101 a and 101 b, so as to cover middle portions ofthe semiconductor fins 101 a and 101 b. In some embodiments, the gatestack 104 a includes a gate dielectric layer 102 a and a gate electrode108 a. Similarly, the gate stack 104 b includes a gate dielectric layer102 b and a gate electrode 108 b. In alternative embodiments, the gatestacks 104 a and 104 b further include spacers 112 a and 112 brespectively.

In alternative embodiments, the gate stack 104 a or 104 b may furtherinclude interfacial layers (ILs) on the semiconductor fin 101 a or 101b. In other words, the gate dielectric layer 102 a or 102 b is formedbetween the IL and the gate electrode 108 a or 108 b respectively. Insome embodiments, the IL includes a dielectric material, such as siliconoxide or silicon oxynitride. The IL is formed by performing a thermaloxidation process, a chemical vapor deposition (CVD) process, or anatomic layer deposition (ALD) process. It should be noted that thedetail described below with respect to the elements of the gate stacks104 a may also apply to the elements of the gate stack 104 b, and thusthe description of the elements in the gate stack 104 b is omitted.

The gate dielectric layer 102 a is formed to cover portions of thesemiconductor fins 101 a. In some embodiments, the gate dielectric layer102 a includes silicon oxide, silicon nitride, silicon oxynitride,high-k dielectric materials, or a combination thereof. The high-kdielectric materials are generally dielectric materials with adielectric constant higher than 4. The high-k dielectric materialsinclude metal oxide. In some embodiments, examples of the metal oxideused as the high-k dielectric materials include oxides of Li, Be, Mg,Ca, Sr, Sc, Y, Zr, Hf, Al, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er,Tm, Yb, Lu, or a combination thereof. The gate dielectric layer 102 a isformed by performing a thermal oxidation process, a CVD process, an ALDprocess, or a combination thereof.

The gate electrode 108 a is formed on the gate dielectric layer 102 a.In some embodiment, the gate electrode 108 a serves as dummy gateelectrode, and the gate electrode 108 a is made of polysilicon. A metalgate (also referred to as “replacement gates”) may replace the dummygate electrode in subsequent steps. The replacing step will be discussedin greater detail in subsequent paragraphs.

In some embodiments, lightly doped source and drain (LDD) regions (notshown) are formed in the semiconductor fins 101 a and 101 b beside thegate stacks 104 a and 104 b. The LDD regions are formed by performing anion implantation process, for example. In some embodiments, thesemiconductor fin 101 a in the region R1 is doped with p-type dopants,and the semiconductor fin 101 b in the region R2 is doped with n-typedopants.

As shown in FIG. 2B, the spacers 112 a are formed over sidewalls of thegate electrode 108 a. The spacers 112 a are formed of silicon oxide,silicon nitride, silicon oxynitride, silicon carbide, SiCN,fluoride-doped silicate glass (FSG), low-k dielectric materials (such asor SiCON), or a combination thereof. It should be noted that the low-kdielectric materials are generally dielectric materials having adielectric constant lower than 3.9. The spacers 112 a may have amulti-layer structure including one or more liner layers. The linerlayer includes a dielectric material such as silicon oxide, siliconnitride, and/or other suitable materials. The formation of the spacers112 a may be achieved by depositing a suitable dielectric material andanisotropically etching off the dielectric material.

As shown in FIG. 2B, recesses 128 a and 128 b are formed within thesemiconductor fin 101 a and 101 b beside the gate stacks 104 a and 104 bby removing a portion of the semiconductor fins 101 a and 101 b atlocations intended for source and drain regions. In some embodiments,the formation of the recesses 128 a and 128 b includes performing afirst etching process and a second etching process. Here, the firstetching process refers to a trench etching process such as ananisotropic etching process, and the second etching process refers to alateral etching process such as an isotropic etching process.

At Step S14 in FIG. 1 and as shown in FIG. 2B, a strain layer 130 a isformed in recess 128 a of the semiconductor fin 101 a in the region R1.The strain layer 130 a is formed at sides of the gate stack 104 a. Alattice constant of the strain layer 130 a is different from a latticeconstant of the substrate 100, and portions of the semiconductor fin 101a covered by the gate stack 104 a is strained or stressed to enhancecarrier mobility and performance of the FinFETs. In one embodiment, thestrain layer 130 a, such as SiGe, are utilized for hole mobilityenhancement of the p-type FinFET to be formed in the region R1.

In addition, a strain layer 130 b is formed in the semiconductor fin 101b in the region R2. A lattice constants of the strain layer 130 b isdifferent from the lattice constant of the substrate 100, and portionsof the semiconductor fin 101 b covered by the gate stack 104 b isstrained or stressed to enhance the carrier mobility and performance ofthe FinFETs. In one embodiment, the strain layer 130 b such as SiC orSiP, are utilized for electron mobility enhancement of the n-type FinFETto be formed in the region R2.

In some embodiments, the strain layers 130 a and 130 b are formedthrough epitaxial growth. In some embodiments, the epitaxial growthtechnology includes performing a low-pressure CVD (LPCVD) process, anatomic layer CVD (ALCVD) process, an ultrahigh vacuum CVD (UHVCVD)process, a reduced pressure CVD (RPCVD) process, a molecular beamepitaxy (MBE) process, a metalorganic vapor phase epitaxy (MOVPE)process or a combination thereof. Alternatively, the epitaxial growthtechnology utilizes a cyclic deposition-etch (CDE) epitaxy process or aselective epitaxial growth (SEG) process to form the strained materialof high crystal quality. In some embodiments, a material of the strainlayers 130 a includes a p-type dopant (such as boron or BF₂ ⁺) dopedtherein formed through selective epitaxial growth by performing in-situdoping, and a material of the strain layer 130 b includes an n-typedopant (such as phosphorus or arsenic) doped therein formed throughselective epitaxial growth by performing in-situ doping.

At Step S16 in FIG. 1 and as shown in FIG. 2C, an etch stop layer 114 isformed over the substrate 100. In some embodiments, the etch stop layer114 may be referred to as a contact etch stop layer (CESL). The etchstop layer 114 includes silicon nitride, carbon-doped silicon nitride,or a combination thereof. In some embodiments, the etch stop layer 114is deposited by performing a CVD process, a high-density plasma (HDP)CVD process, a sub-atmospheric CVD (SACVD) process, a molecular layerdeposition (MLD) process, or other suitable processes. In someembodiments, before the etch stop layer 114 is formed, a buffer layer(not shown) may be formed over the substrate 100. In an embodiment, thebuffer layer is an oxide such as silicon oxide. However, othercompositions may be possible. In some embodiments, the buffer layer isdeposited by performing a CVD process, a HDPCVD process, a SACVDprocess, an MLD process, or other suitable processes.

At Step S18 in FIG. 1 and as shown in FIG. 2C, a dielectric layer 116 isformed over the etch stop layer 114. In some embodiments, the dielectriclayer 116 may be referred to as an interlayer dielectric layer (ILD).The dielectric layer 116 includes silicon oxide, silicon nitride,silicon oxynitride, phosphosilicate glass (PSG), borophosphosilicateglass (BPSG), spin-on glass (SOG), fluorinated silica glass (FSG),carbon doped silicon oxide (e.g., SiCOH), polyimide, and/or acombination thereof. In some other embodiments, the dielectric layer 116includes low-k dielectric materials. It should be noted that the low-kdielectric materials are generally dielectric materials having adielectric constant lower than 3.9. Examples of low-k dielectricmaterials include BLACK DIAMOND® (Applied Materials of Santa Clara,Calif.), Xerogel, Aerogel, amorphous fluorinated carbon, Parylene, BCB(bis-benzocyclobutenes), Flare, SILK® (Dow Chemical, Midland, Mich.),hydrogen silsesquioxane (HSQ) or fluorinated silicon oxide (SiOF),and/or a combination thereof. It is understood that the dielectric layer116 may include one or more dielectric materials and/or one or moredielectric layers. In some embodiments, the dielectric layer 116 isformed to have a suitable thickness by performing a flowable CVD (FCVD)process, a CVD process, a HDPCVD process, a SACVD process, a spin-onprocess, a sputtering process, or other suitable processes.

At Step S20 in FIG. 1 and as shown in FIG. 2D, the dielectric layer 116and the etch stop layer 114 are partially removed such that top surfacesof the gate stacks 104 a and 104 b are exposed, and a dielectric layer116 a and an etch stop layer 114 a are formed. The process of removing aportion of the dielectric layer 116 and a portion of the etch stop layer114 is achieved by a chemical mechanical polishing (CMP) process, anetching process, or other suitable processes.

At Step S22 in FIG. 1 and as shown in FIGS. 2D and 2E, a metalreplacement process is performed. In some embodiments, the gateelectrodes 108 a and 108 b are dummy gate electrodes, and are replacedrespectively by gate electrodes 120 a and 120 b. Specifically, materialsof the gate electrodes 108 a and 108 b are polysilicon and the materialsof the gate electrodes 120 a and 120 b include metal-containingconductive layers. At least one of the metal-containing conductivelayers includes a barrier, a work function layer, a seed layer, anadhesion layer, a barrier layer, a filter layer, a metal filling layeror a combination thereof. The metal-containing conductive layers includeAl, Cu, W, Ti, Ta, Ag, Ru, Mn, Zr, TiAl, TiN, TaN, WN, TiAlN, TaC, TaCN,TaSiN, TiSiN, NiSi, CoSi, or a combination thereof, for example. Themetal-containing conductive layers are formed by formingmetal-containing conductive material layers, and a chemical mechanicalpolishing (CMP) process, an etching process, or other suitableprocesses.

In some embodiments, the gate electrode 120 a includes a work functionlayer 122 a and a metal filling layer 124 a for the p-type FinFET. Thegate electrode 120 b includes a work function layer 122 b and a metalfilling layer 124 b for the n-type FinFET. As shown in FIG. 2E, the workfunction layers 122 a and 122 b have a U-shape cross section,respectively. The metal filling layer 124 a and 124 b are disposed overthe work function layers 122 a and 122 b, and are surrounded by the workfunction layers 122 a and 122 b, respectively. In the case, the gatedielectric layer 102 a and the gate electrode 120 a constitute a gatestructure 140 a for the p-type FinFET, while the gate dielectric layer102 b and the gate electrode 120 b constitute of a gate structure 140 bfor the n-type FinFET. The spacers 112 a are formed over the oppositesidewalls of the gate structure 140 a; while the spacers 112 b areformed over the opposite sidewalls of the gate structure 140 b.

FIG. 3A is a perspective view of a FinFET in accordance with someembodiments. FIG. 3B is a cross-sectional view of the FinFET taken alongthe line I-I′ of FIG. 3A. In some embodiments, the FinFET illustrated inFIG. 3A is referred as the FinFET over the substrate 100 in the regionR1 and/or region R2 illustrated in FIG. 2E.

As shown in FIG. 3A and FIG. 3B, a FinFET includes a substrate 100, aplurality of isolation structures 201, a gate structure 140, and aplurality of strain layers 130. The substrate 100 has a plurality ofsemiconductor fins 101. The isolation structures 201 are located overthe substrate 100 to isolate the semiconductor fins 101. Thesemiconductor fins 101 and the isolation structures 201 both extendalong the Y direction. In addition, the semiconductor fins 101 protrudefrom the isolation structures 201. The gate structure 140 extend alongthe X direction. The gate structure 140 is disposed across portions ofthe semiconductor fins 101 and portions of the isolation structures 201.In detail, the gate structure 140 includes a gate dielectric layer 102and a gate electrode 120 over the gate dielectric layer 102. The gateelectrode 120 includes a work function layer 122 and a metal fillinglayer 124 surrounded by the work function layer 122. The strain layers130 formed at two sides of the gate structure 140 (as shown in FIG. 3B)are referred as source and/or drain (S/D) regions. The FinFET furtherincludes spaces 112, an etch stop layer 114 a, and a dielectric layer116 a. The spaces 112 are disposed over the opposite sidewalls of thegate dielectric layer 102. The etch stop layer 114 a is formed to coverthe spaces 112, the strain layers 130, and the isolation structures 201.The dielectric layer 116 a is formed over the etch stop layer 114 a.

FIGS. 4A to 4F are enlarged exemplary cross-sectional viewscorresponding to an area A of FIG. 3B for various transistors inaccordance with some embodiments. FIGS. 4A to 4C show exemplarystructures of the work function layer corresponding to area A of FIG. 3Bfor n-type transistors with different threshold voltages. In somealternatively embodiments, FIGS. 4D to 4E show exemplary structures ofthe work function layer corresponding to area A of FIG. 3B for p-typetransistors with different threshold voltages.

FIG. 4A is exemplary cross-sectional view illustrating the structure ofthe work function layer of a first n-type FinFET N1 with a thresholdvoltage Vn1 (e.g., ultra-low voltage). FIG. 4B is exemplarycross-sectional view illustrating the structure of the work functionlayer of a second n-type FinFET N2 with a threshold voltage Vn2 (e.g.,low-voltage). FIG. 4C is exemplary cross-sectional view illustrating thestructure of the work function layer of a third n-type FinFET N3 with athreshold voltage Vn3 (e.g., standard voltage). Here, Vn1<Vn2<Vn3.

As shown in FIG. 4A, the structure of the work function layer of thefirst n-type FinFET N1 includes a first conductive layer 125, a firstlayer 202, and a second conductive layer 126 between the gate dielectriclayer 102 and the metal filling layer 124.

In some embodiments, the first conductive layer 125 functions as abarrier or protective layer in the subsequent etching processes, isformed over the gate dielectric layer 102. In some embodiments, thefirst conductive layer 125 includes one or more layers of Ti, Ta, TiNand TaN. For example, the first conductive layer 125 may be a single TaNlayer, or a composite layer of TaN layer formed over a TiN layer. Thefirst conductive layer 125 is formed by performing a CVD process, an ALDprocess, or other suitable process. A thickness of the first conductivelayer 125 is in a range from 5 Å to 25 Å, in some embodiments.

The first layer 202 is also referred as a first work function layer 200a. In some embodiments, the first layer 202 includes TiAl, TaAl, HfAl,or a combination thereof. In some embodiments, the first layer 202includes one or more layers. The first layer 202 is formed by performinga CVD process, an ALD process, or other suitable process. A thickness ofthe first layer 202 is in a range from 15 Å to 50 Å, in someembodiments, and in a range from 25 Å to 40 Å in other embodiments.

In some embodiments, the second conductive layer 126 functions as anadhesive layer for the metal filling layer 124, and includes one or morelayers of Ti, Ta, TiN and TaN. In one embodiment, a TiN layer is used asthe second conductive layer 126. The second conductive layer 126 isformed by performing a CVD process, an ALD process, or other suitableprocess, for example. A thickness of the second conductive layer 126 isin a range from 25 Å to 45 Å, in some embodiments.

The material and forming method of the gate dielectric layer 102 and themetal filling layer 124 have been described in the foregoing paragraph.Thus, details thereof are omitted here.

As shown in FIG. 4B, the arrangement, material and forming method of thesecond n-type FinFET N2 are similar to the arrangement, material andforming method of the first n-type FinFET N1. Thus, details thereof areomitted here. A difference therebetween lies in that the structure ofthe work function layer of the second n-type FinFET N2 further includesa second layer 204. The second layer 204 is formed between the firstconductive layer 125 and the first layer 202. In some embodiments, thefirst layer 202 and the second layer 204 are referred as a second workfunction layer 200 b.

In some embodiment, the second layer 204 includes one or more layers ofTi, Ta, TiN and TaN. In one embodiment, the second layer 204 includes aTiN layer. In some embodiments, the second layer 204 and the first layer202 include different materials. For example, the second layer 204includes a base material and a dopant doped in the base material. Thebase material of the second layer 204 includes a metal nitride such asTiN, TaN or a combination thereof. The dopant includes a metal atom suchas Al, Ta, W, or a combination thereof. The second layer 204 may beformed by doping Al elements into a TiN layer to form a TiAlN layer, forexample. However, the embodiments of the present invention are notlimited thereto. In some alternatively embodiments, the base materialincludes TiN, TaN, and the second layer 204 includes TiAlN, TaAlN, TiWN,TiN—Ta, or a combination thereof. A thickness of the second layer 204 isin a range from 7 Å to 15 Å, in some embodiments. The second layer 204may be formed by performing an ALD process, will be described in detailin the following paragraphs.

As shown in FIG. 4C, the arrangement, material and forming method of thethird n-type FinFET N3 are similar to the arrangement, material andforming method of the second n-type FinFET N2. Thus, details thereof areomitted here. A difference therebetween lies in that the structure ofthe work function layer of the third n-type FinFET N3 further includes athird layer 206, and the third layer 206 is formed between the firstconductive layer 125 and the second layer 204. In some embodiments, thefirst layer 202, the second layer 204 and the third layer 206 arereferred as a third work function layer 200 c. In some embodiment, thethird layer 206 includes one or more layers of Ti, Ta, TiN and TaN. Inone embodiment, the third layer 206 includes a TiN layer. In someembodiments, the third layer 206 and the first layer 202 includedifferent materials. For example, the third layer 206 includes a basematerial and a dopant doped in the base material. The base material ofthe third layer 206 may be the same as or different from the basematerial of the second layer 204. The base material of the third layer206 includes a metal nitride such as TiN, TaN or a combination thereof.The dopant of the third layer 206 may be the same as or different fromthe dopant of the second layer 204. The dopant of the third layer 206includes a metal atom such as Al, Ta, W, or a combination thereof. Insome alternatively embodiments wherein the base material includes TiN,TaN, or a combination thereof, and the third layer 206 includes TiAlN,TaAlN, TiWN, TiN—Ta, or a combination thereof. The third layer 206 maybe formed by performing an ALD process, will be described in detail inthe following paragraphs. A thickness of the third layer 206 is in arange from 7 Å to 15 Å, in some embodiments. Two adjacent layers may betreated as one “thick” layer. For example, when the third layer 206 andthe second layer 204 are formed of the same material (e.g., TiAlN), thethird layer 206 and the second layer 204 may be treated as one “thick”layer (e.g., a thick TiAlN layer).

As shown in FIGS. 4A to 4C, the base materials of the second layer 204and the third layer 206, and the dopants and the dopant concentrationsof the second layer 204 to the third layer 206 may be the same ordifferent. On the other hand, the thicknesses of the first to third workfunction layers 200 a, 200 b, 200 c are different. In some embodiments,the thickness of the first work function layer 200 a for the firstn-type FinFET N1 is the smallest, while the thickness of the third workfunction layer 200 c for the third n-type FinFET N3 is the largest,among the first to third n-type FinFETs N1, N2 and N3. As set forthabove, by adjusting the thickness of the work function layer, thresholdvoltages (Vt) of the first to third n-type FinFETs N1, N2 and N3 may beadjusted to 0<Vn1<Vn2<Vn3. That is, the threshold voltages of the firstto third n-type FinFETs N1, N2 and N3 increases as the total thicknessof the first to third work function layers 200 a, 200 b, 200 cincreases.

In some embodiments, at least one of the second work function layer 200b and the third work function layer 200 c includes a base material(e.g., TiN, TaN or a combination thereof) and a dopant doped in the basematerial, and the dopant comprises Al, Ta, W, or a combination thereof.In detail, at least one of the second layer 204 and the third layer 206includes the base material (e.g., TiN, TaN, or a combination thereof)and a dopant doped in the base material. It should be noted that thework function layer having the dopant therein is able to achieve to adesired threshold voltage at a thicker thickness of the work functionlayer compared with the work function layer without the dopant. In otherwords, the work function layer having the dopant is able to prevent thefilm discontinuous and or non-uniform issue due to the ultra-thin fileformation. In the case, the work function layer having the dopant issuitable for the next generation semiconductor device, such as thecritical dimension of 16 nm, 10 nm, 7 nm, 5 nm, 3 nm or less.

In addition, the work function layer having the dopant may function as ablocking layer to prevent diffusion atoms (such as W, Al, F or O)diffusing from the metal filling layer into the gate dielectric layer orthe interfacial layer. The diffusion atoms include metal atoms (such asW, Al) derived from the metal filling layer and/or residues (such as For O) derived from a precursor of the metal filling layer and/or n workfunction layer, respectively. As a result, the issue of thresholdvoltage shift may be prevented or decreased.

FIG. 4D is exemplary cross-sectional view illustrating the structure ofthe work function layer of a first p-type FinFET P1 with a thresholdvoltage Vp1 (e.g., standard voltage). FIG. 4E is exemplarycross-sectional view illustrating the structure of the work functionlayer of a second p-type FinFET P2 with a threshold voltage Vp2 (e.g.,low-voltage). FIG. 4F is exemplary cross-sectional view illustrating thestructure of the work function layer of a third p-type FinFET P3 with athreshold voltage Vp3 (e.g., ultra-low voltage). Here, Vp1<Vp2<Vp3<0 V.

As shown in FIG. 4D, the arrangement, material and forming method of thestructure of the work function layer of the first p-type FinFET P1 aresimilar to the arrangement, material and forming method of the structureof the work function layer of the third n-type FinFET N3. Thus, detailsthereof are omitted here. A difference therebetween lies in that thestructure of the work function layer of the first p-type FinFET P1further includes a fourth layer 208 formed between the first conductivelayer 125 and the third layer 206. In some embodiments, the first layer202, the second layer 204, the third layer 206, and the fourth layer 208are referred as a fourth work function layer 200 d. The fourth layer 208includes one or more layers of Ti, Ta, TiN and TaN. In one embodiment,the fourth layer 208 includes a TiN layer. In some embodiments, thefourth layer 208 and the first layer 202 include different materials.For example, the fourth layer 208 includes a base material and a dopantdoped in the base material. The base material of the fourth layer 208may be the same as or different from the base material of the secondlayer 204. The base material of the fourth layer 208 includes a metalnitride such as TiN, TaN or a combination thereof. The dopant of thefourth layer 208 may be the same as or different from the dopant of thesecond layer 204. The dopant of the fourth layer 208 includes a metalatom such as Al, Ta, W, or a combination thereof. In some embodiments,the base material includes TiN, TaN, or a combination thereof, and thefourth layer 208 may include TiAlN, TaAlN, TiWN, TiN—Ta, or acombination thereof. A thickness of the fourth layer 208 is in a rangefrom 7 Å to 15 Å, in some embodiments. The fourth layer 208 may beformed by performing an ALD process, will be described in detail in thefollowing paragraphs.

As shown in FIG. 4E, the arrangement, material and forming method of thestructure of the work function layer of the second p-type FinFET P2 aresimilar to the arrangement, material and forming method of the structureof the work function layer of the first p-type FinFET P1. Thus, detailsthereof are omitted here. A difference therebetween lies in that thestructure of the work function layer of the second p-type FinFET P2further includes a fifth layer 210 formed between the first conductivelayer 125 and the fourth layer 208. In some embodiments, the first layer202, the second layer 204, the third layer 206, the fourth layer 208,and the fifth layer 210 are referred as a fifth work function layer 200e. The fifth layer 210 includes one or more layers of Ti, Ta, TiN andTaN. In one embodiment, the fifth layer 210 includes a TiN layer. Insome embodiments, the fifth layer 210 and the first layer 202 includedifferent materials. For example, the fifth layer 210 includes a basematerial and a dopant doped in the base material. The base material ofthe fifth layer 210 may be the same as or different from the basematerial of the second layer 204. The base material of the fifth layer210 includes a metal nitride such as TiN, TaN or a combination thereof.The dopant of the fifth layer 210 may be the same as or different fromthe dopant of the second layer 204. The dopant of the fifth layer 210includes a metal atom such as Al, W, Ta, or a combination thereof. Insome embodiments where the base material includes TiN, TaN, or acombination thereof, and the fifth layer 210 may include TiAlN, TaAlN,TiWN, TiN—Ta, or a combination thereof. A thickness of the fifth layer210 is in a range from 7 Å to 15 Å, in some embodiments. The fifth layer210 may be formed by performing an ALD process, will be described indetail in the following paragraphs.

As shown in FIG. 4F, the arrangement, material and forming method of thestructure of the work function layer of the third p-type FinFET P3 aresimilar to the arrangement, material and forming method of the structureof the work function layer of the second p-type FinFET P2. Thus, detailsthereof are omitted here. A difference therebetween lies in that thestructure of the work function layer of the third p-type FinFET P3further includes a sixth layer 212 formed between the first conductivelayer 125 and the fifth layer 210. In some embodiments, the first layer202, the second layer 204, the third layer 206, the fourth layer 208,the fifth layer 210, and the sixth layer 212 are referred as a sixthwork function layer 200 f. The sixth layer 212 includes one or morelayers of Ti, Ta, TiN and TaN. In one embodiment, the sixth layer 212includes a TiN layer. In some embodiments, the sixth layer 212 and thefirst layer 202 include different materials. For example, the sixthlayer 212 includes a base material and a dopant doped in the basematerial. The base material of the sixth layer 212 may be the same as ordifferent from the base material of the second layer 204. The basematerial of the sixth layer 212 includes a metal nitride such as TiN,TaN or a combination thereof. The dopant of the sixth layer 212 may bethe same as or different from the dopant of the second layer 204. Thedopant of the fourth layer 208 includes a metal atom such as Al, Ta, W,or a combination thereof. In some embodiments where the base materialincludes TiN, TaN, or a combination thereof, and the sixth layer 212 mayinclude TiAlN, TaAlN, TiWN, TiN—Ta, or a combination thereof. Athickness of the sixth layer 212 is in a range from 7 Å to 15 Å, in someembodiments. The sixth layer 212 may be formed by performing an ALDprocess, will be described in detail in the following paragraphs.

As shown in FIGS. 4D to 4F, the base materials, the dopants, the dopantconcentrations of the fourth layer 208 to the sixth layer 212 may be thesame or different.

As shown in FIGS. 4D to 4F, at least two adjacent layers made of thesame material may be treated as one “thick” WFM layer. For example, whenthe sixth (fifth, fourth or third) to second layers are formed of thesame material (e.g., TiAlN), the sixth (fifth, fourth or third) tosecond layers may be treated as one “thick” layer (e.g., a thick TiAlNlayer).

As shown in FIGS. 4D to 4F, the thicknesses of the fourth to sixth workfunction layers 200 d, 200 e, 200 f are different. Further, thethickness of the sixth work function layer 200 f for the third p-typeFinFET P3 is larger than the thickness of the fourth work function layer200 d for the first p-type FinFET P1, and larger than the thickness ofthe fifth work function layer 200 e for the second p-type FinFET P2. Inother words, the thickness of the fourth work function layer 200 d forthe first p-type FinFET P1 is the smallest, while the thickness of thesixth work function layer 200 f for the third p-type FinFET P3 is thelargest, among the first to third p-type FinFETs P1, P2, P3.

As shown in FIGS. 4A to 4F, in some embodiments, the first layer 202 isdifferent from the base material of the second layer 204 to the sixthlayer 212. The base materials of the second layer 204 to the sixth layer212 may be the same or different, and the dopants and the dopantconcentrations of the second layer 208 to the sixth layer 212 may be thesame or different. In other words, the second to sixth work functionlayers 200 a to 200 f may have the same or different base materials, andthe second to sixth work function layers 200 b to 200 f may have thesame or different dopants and dopant concentrations. In other words, insome embodiments, at least one of the constituent atoms of the secondlayer 204 to the sixth layer 212 are the same as at least one of theconstituent atoms of the first layer 202. In some other embodiments, atleast two of the constituent atoms of the second layer 208 to the sixthlayer 212 are the same as at least two of the constituent atoms of thefirst layer 202. In some other embodiments, at least two of theconstituent atoms of the second layer 204 to the sixth layer 212 are thesame as all of the constituent atoms of the first layer 202. Forexample, the second layer 204 to the sixth layer 212 are TiAlN, and thefirst layer 202 are TiAl.

As shown in FIGS. 4A to 4F, the total thickness of the work functionlayer increases in the order of the first to sixth work function layers200 a to 200 f. As set forth above, by adjusting the thickness of thework function layer, threshold voltages (Vt) of the first to thirdn-type FinFETs N1, N2 and N3 may be adjusted to 0<Vn1<Vn2<Vn3, andthreshold voltages (Vt) for the first to third p-type FinFETs P1, P2, P3may be adjusted to Vp1>Vp2>Vp3. That is, the threshold voltages of thefirst to third n-type FinFETs N1, N2 and N3 increases as the totalthickness of the first to third work function layers 200 a, 200 b, 200 cincreases, while the threshold voltage of the first to third p-typeFinFETs P1, P2, P3 decrease as the total thickness of the fourth tosixth work function layers 200 d, 200 e, 200 f increases.

As shown in FIGS. 4A-4F, in some embodiments, at least one of the secondto six layers 204, 206, 208, 210 and 212 includes the base material andthe dopant doped in the base material, and the dopant includes a metalatom such as Al, Ta, W, or a combination thereof. The second to sixlayers 204, 206, 208, 210 and 212 may include different materials (e.g.,TiAlN and TiN) or the same material (e.g., TiAlN or TiN). In somealternatively embodiments, all of the second to six layers 204, 206,208, 210 and 212 includes the base material and the dopant doped in thebase material, and the dopant includes Al, Ta, W, or a combinationthereof. In some embodiments, the work function layers 200 b to 200 ffor the second to third n-type FinFETs N2, N3 and the first to thirdp-type FinFETs P1, P2, P3 are made of the same material with differentthicknesses. In the case, the thickness of the first work functionlayers 200 a for the first n-type FinFET N1 is smallest, while thethickness of the sixth work function layers 200 f for the third p-typeFinFET P3 is the largest.

In addition, in some embodiments, the semiconductor device includes oneor more n-type FinFETs with different threshold voltages and/or one ormore p-type FinFETs with different threshold voltages.

In some embodiments, the three n-type FinFETs and three p-type FinFETsare disposed in a core area or an I/O (input/output) area, in otherembodiments. A thickness of the gate dielectric layer in the core areais less than a thickness of the gate dielectric layer in the I/O area.

FIGS. 5A to 5D are exemplary cross-sectional views illustrating variousstages of the sequential fabrication process of a gate structure inaccordance with some embodiments. FIG. 6 is a flow chart illustrating amethod of manufacturing a gate structure of a semiconductor device inaccordance with some embodiments. In FIGS. 5A to 5D, the fabricationprocesses for metal gate structures for n-type FinFETs are illustrated,but substantially the same fabrication processes may be applied top-type FinFETs.

At Steps S102 and S104 in FIG. 6 and as shown in FIG. 5A, the gatedielectric layer 102 is formed over the substrate (not shown) and thefirst conductive layer 125 is formed over the gate dielectric layer 102.At Steps S106 and S108 in FIG. 6 and as shown in FIGS. 5B to 5D, thework function layer 200 a, 200 b and 200 c for the first to third n-typeFinFETs N1, N2 and N3 are formed over the first conductive layer 125 andthe second conductive layer 126 is formed over the work function layer200 a, 200 b and 200 c.

In detail, as shown in FIG. 5A, the third layer 206 is formed over thefirst conductive layer 125 for the first to third n-type FinFETs N1, N2and N3 (as shown in FIG. 5D). The third layer 206 formed on the firstconductive layer 125 for the first and second n-type FinFETs N1 and N2are removed by dry etching. By the dry etching, the first conductivelayers 125 for the first and second n-type FinFETs N1 and N2 areexposed.

As shown in FIG. 5B, the second layer 204 is formed over the firstconductive layer 125 for the first and second n-type FinFETs N1 and N2and over the third layer 206 for the third n-type FinFET N3.Subsequently, the second layer 204 formed on the first conductive layer125 for the first n-type FinFET N1 is removed by dry etching. By the dryetching, the first conductive layer 125 for the first n-type FinFET N1is exposed.

As shown in FIG. 5C, the first layer 202 is formed over the firstconductive layer 125 for the first n-type FinFET N1 and over the secondlayer 204 for the second and third n-type FinFETs N2 and N3.Subsequently, the second conductive layer 126 and the metal fillinglayer 124 are formed over the first layer 202, as shown in FIG. 5D.

In some embodiment, at least one of the second layer 204 and the thirdlayer 206 includes the base material (e.g., TiN, TaN, or a combinationthereof) and a dopant (e.g., Al, Ta, W, or a combination thereof) dopedin the base material. For example, the Al dopants are doped into the TiNlayer to form the TiAlN layer by the ALD process as an exemplaryembodiment.

The ALD process includes repeating deposition loops, and each loopincluding a first cycle and a second cycle, in some embodiments. Indetail, the first cycle is performed to form a metal nitride layer(e.g., the TiN layer). The first cycle includes using sources includinga nitrogen source and Ti source. The nitrogen source includes NH₃, N₂,and N₂O, and the Ti source includes TiCl₄. Specifically, the first cycleincludes following steps of introducing the nitrogen precursor for 0.2seconds to 1 seconds, performing a first purge process for 0.2 secondsto 1 seconds, introducing the metal precursor for 0.1 seconds to 1seconds, and performing a second purge process for 0.2 seconds to 1seconds. In some embodiments, the first and second purge processes maybe used to maintain cleanliness in the deposition chamber (e.g., toremove contaminants, particles, and other undesired matter from thedeposition chamber) and to control a flow of gases in the depositionchamber. The controlling of the flow of gases may be used, for example,to remove precursor materials from the deposition chamber.

After the metal nitride layer (e.g., the TiN layer) is formed, thesecond cycle are performed by introducing an aluminum precursor on asurface of the metal nitride layer. In some embodiments, the aluminumprecursor may be an organic precursor (e.g., trimethylaluminium (TMA))or a non-organic precursor (e.g., aluminium chloride (AlCl₃)).Specifically, the second cycle includes following steps of introducingthe aluminum precursor for 0.1 seconds to 1 seconds and performing athird purge process for 0.2 seconds to 1 seconds. After the second cycleare performed, Al atoms (or element Al) are doped or introduced into theTiN layer to form a first TiAlN layer as a first layer of the workfunction layer. After the loop including the first cycle and the secondcycle is accomplished, the first cycle and the second cycle are repeatedto form a second TiAlN layer, and/or a third TiAlN layer and so on untila thickness of the work function layer is achieved.

In some embodiments, each of the loops includes performing the firstcycle once and performing the second cycle once. In some otherembodiments, each of the loops includes performing the first cycle onceand performing the second cycle twice for adjusting Al content. However,the disclosure is not limited thereto. In so alternative embodiments,the number of performing the first cycle and the number of performingthe second cycle may be the same or different, in other embodiments.

In some alternatively embodiments, each loop includes the first cycleand a third cycle. The steps of the first cycle have been described inthe foregoing paragraph. Thus, details thereof are omitted here. Afterthe metal nitride layer (e.g., the TiN layer) is formed, the third cycleare performed by introducing another nitrogen precursor (e.g., NH₃ asnitrogen source) and an aluminum precursor (e.g., TMA or a AlCl₃) on thesurface of the metal nitride layer to form a AlN layer over the TiNlayer. In other words, the AlN layer and the TiN layer are in contactwith each other to from a TiAlN layer as the work function layer. Afterthe loop including the first cycle and the third cycle is accomplished,the first cycle and the third cycle are repeated to achieve a thicknessof the work function layer. In some alternative embodiments, the numberof performing the first cycle and the number of performing the thirdcycle may be the same or different, in other embodiments.

In some embodiments, the time of introducing the aluminum precursor maybe less than 1 second which is referred as a short pulse. When the Alprecursor is introduced with the short pulse, the formed TiAlN layer hasbetter uniformity, better step coverage, and higher wafer per hour(WPH). That is, the process time of performing the ALD process with theshort pulse is shortened, so that the throughput of forming the gatestructure is faster. In some alternatively embodiments, parameters(e.g., temperature, power, flow rate or the like) of the ALD process maybe adjusted based on the design and the requirements of the devices.

In addition, in some embodiments, the tensile stress of the workfunction layer decreases as the aluminum content increases. When the Alcontent increases, the crystal plane orientation of the TiAlN layerchanges from <111> to <200>. The <200> crystal plane orientation hasless tensile stress than the <111> crystal plane orientation. That is,the work function layer having the Al dopant is able to release thetensile stress of the whole gate structure and avoid the gate structurebending issue. In some alternatively embodiments, the work functionlayer having the dopant may be amorphous. In one embodiment, the Alcontent of the work function layer (e.g., the TiAlN layer) is in therange of 10 wt % and 18 wt %. However, the embodiments of the presentinvention are not limited thereto. In another embodiment, a ratio of theTiN to Al of the TiAlN layer is 1:1; while the ratio of the TiN to AlNof the TiAlN layer is 1:2 to 2:1 (i.e., 1:2, 1:1, or 2:1), in otherembodiments.

Although the above steps are doping the Al dopants into the TiN layer toform the TiAlN layer by the ALD process as an exemplary embodiment, itshould not limit various embodiments of the present disclosure. In somealternative embodiments, the dopants may include Al, Ta, W or acombination thereof, and the base material comprises TiN, TaN, or acombination thereof. Therefore, the second layer or the third layerformed by doping the dopants into the base material by the ALD processmay include TiAlN, TaAlN, TiWN, TiN—Ta, or a combination thereof. In oneembodiment, the first cycle is performed to introduce a nitrogenprecursor (e.g., NH₃, N₂, N₂O, or a combination thereof) and a firstmetal precursor (e.g., TiCl₄, TaCl₅, or a combination thereof) to form ametal nitride layer (e.g., TiN, TaN, or a combination thereof). A secondcycle is then performed to introduce a second metal precursor (e.g.,TMA, AlCl₃, TIBA, WF₆, TaCl₅, or a combination thereof) on a surface ofthe metal nitride layer. In another embodiment, a third cycle isperformed to introduce a second nitrogen precursor (e.g., NH₃, N₂, N₂O,or a combination thereof) and a second metal precursor (e.g., TMA,AlCl₃, TIBA, WF₆, TaCl₅, or a combination thereof) on a surface of themetal nitride layer after the first cycle is performed.

In accordance with some embodiments of the present disclosure, asemiconductor device includes a first n-type FinFET and a second n-typeFinFET. The first FinFET includes a first work function layer. The firstwork function layer includes a first portion of a first layer. Thesecond n-type FinFET includes a second work function layer. The secondwork function layer includes a second portion of the first layer and afirst portion of a second layer underlying the second portion of thefirst layer. A thickness of the first work function layer is less than athickness of the second work function layer.

In accordance with alternative embodiments of the present disclosure, asemiconductor device includes a n-type FinFET. The n-type FinFETincludes a gate structure between a pair of source and/or drain (S/D)regions. The gate structure includes a gate dielectric layer; a metalfilling layer over the gate dielectric layer; and a work function layerbetween the gate dielectric layer and the metal filling layer. The workfunction layer is selected from a material in which a threshold voltageof the n-type FinFET increases as a thickness of the work function layerincreases.

In accordance with yet alternative embodiments of the presentdisclosure, a method of manufacturing a semiconductor device includes:providing a substrate having a first region and a second region; forminga first conductive layer over the substrate in the first and secondregions; forming a bottom material over the first conductive layer bydoping a dopant in a base material in an atomic layer deposition (ALD)method, wherein the dopant comprises Al, Ta, W or a combination thereof;removing a portion of the bottom material to expose the first conductivelayer in the first region and form a bottom layer over the firstconductive layer in the second region; forming a top material to coverthe first conductive layer in the first region and the bottom layer inthe second region, wherein a first portion of the top material in thefirst region is formed as a first work function layer for a firstFinFET, and a second portion of the top material in the second regionand the bottom layer there-below are formed as a second work functionlayer for a second FinFET; and forming a second conductive layer tocover the first and second work function layers.

The foregoing outlines features of several embodiments so that thoseskilled in the art may better understand the aspects of the presentdisclosure. Those skilled in the art should appreciate that they mayreadily use the present disclosure as a basis for designing or modifyingother processes and structures for carrying out the same purposes and/orachieving the same advantages of the embodiments introduced herein.Those skilled in the art should also realize that such equivalentconstructions do not depart from the spirit and scope of the presentdisclosure, and that they may make various changes, substitutions, andalterations herein without departing from the spirit and scope of thepresent disclosure.

What is claimed is:
 1. A semiconductor device, comprising: a firstn-type transistor comprising a first work function layer, the first workfunction layer comprising a first underlying layer; and a second n-typetransistor comprising a second work function layer, the second workfunction layer comprising a second underlying layer, wherein the firstand second underlying layers each comprises a metal nitride layer withat least two kinds of metals, and a thickness of the first underlyinglayer is greater than a thickness of the second underlying layer.
 2. Thesemiconductor device of claim 1, wherein the at least two kinds ofmetals comprise a first metal comprising Ti, Ta, or a combinationthereof, and a second metal comprising Al, Ta, W, or a combinationthereof.
 3. The semiconductor device of claim 2, wherein a content ofthe second metal in the first underlying layer or the second underlyinglayer is in the range of 10 wt % and 18 wt %.
 4. The semiconductordevice of claim 1, wherein the first work function layer furthercomprises a first overlying layer overlaying the first underlying layer,the second work function layer further comprises a second overlyinglayer overlaying the second underlying layer, and the first and secondoverlying layers each comprises TiAl, TaAl, HfAl, or a combinationthereof.
 5. The semiconductor device of claim 1, further comprising athird n-type transistor comprising a third work function layer, whereinthird work function layer comprises a third overlying layer, and thethird overlying layer comprises TiAl, TaAl, HfAl, or a combinationthereof.
 6. The semiconductor device of claim 5, wherein the firstn-type transistor has a threshold voltage Vn1, the second n-typetransistor has a threshold voltage Vn2, the third n-type transistor hasa threshold voltage Vn3, and 0<Vn3<Vn2<Vn1.
 7. The semiconductor deviceof claim 1, further comprising: a first p-type transistor with athreshold voltage Vp1 comprising a fourth work function layer; a secondp-type transistor with a threshold voltage Vp2 comprising a fifth workfunction layer; and a third p-type transistor with a threshold voltageVp3 comprising a sixth work function layer, wherein Vp1<Vp2<Vp3<0, athickness of the sixth work function layer is greater than a thicknessof the fifth work function layer, and the thickness of the fifth workfunction layer is greater than a thickness of the fourth work functionlayer.
 8. A semiconductor device, comprising: a p-type FinFET comprisinga first gate structure between a pair of first source and/or drain (S/D)regions, the first gate structure comprising: a first gate dielectriclayer; a first metal filling layer over the first gate dielectric layer;and a first work function layer between the first gate dielectric layerand the first metal filling layer, wherein the first work function layeris selected from a first material in which a threshold voltage of thep-type FinFET decreases as a thickness of the first work function layerincreases.
 9. The semiconductor device of claim 8, further comprising abarrier layer and an adhesive layer, wherein the first work functionlayer is disposed between the barrier layer and the adhesive layer. 10.The semiconductor device of claim 9, wherein the first work functionlayer comprises a first layer and a second layer underlying the firstlayer, the first layer comprises TiAl, TaAl, HfAl, or a combinationthereof, and the second layer comprises a metal nitride layer with atleast two kinds of metals.
 11. The semiconductor device of claim 10,wherein the first layer is between and in contact with the second layerand the adhesive layer, and the second layer is between and in contactwith the barrier layer and the first layer.
 12. The semiconductor deviceof claim 10, wherein the at least two kinds of metals comprises a firstmetal comprising Ti, Ta, or a combination thereof, and a second metalcomprising Al, Ta, W, or a combination thereof.
 13. The semiconductordevice of claim 8, further comprising: a n-type FinFET comprising asecond gate structure between a pair of second source and/or drain (S/D)regions, the second gate structure comprising: a second gate dielectriclayer; a second metal filling layer over the second gate dielectriclayer; and a second work function layer between the second gatedielectric layer and the second metal filling layer, wherein the secondwork function layer is selected from a second material in which athreshold voltage of the n-type FinFET increases as a thickness of thesecond work function layer increases.
 14. A method of manufacturing agate structure for a semiconductor device, comprising: providing asubstrate at least having a first region and a second region; forming afirst gate structure having a first work function layer over the firstregion for a first n-type transistor; and forming a second gatestructure having a second work function layer over the second region fora second n-type transistor, wherein the first work function layercomprises a first underlying layer and the second work function layercomprises a second underlying layer having a thickness less than athickness of the first underlying layer, wherein each of the first andsecond underlying layers is doped with a dopant in a base material by anatomic layer deposition (ALD) method, and the dopant comprises Al, Ta, Wor a combination thereof.
 15. The method of claim 14, wherein the firstwork function layer further comprises a first overlying layer formed onthe first underlying layer, and the first overlying layer comprisesTiAl, TaAl, HfAl, or a combination thereof.
 16. The method of claim 14,wherein the second work function layer further comprises a secondoverlying layer formed on the second underlying layer, and the secondoverlying layer comprises TiAl, TaAl, HfAl, or a combination thereof.17. The method of claim 14, wherein doping the each of the first andsecond underlying layers with the dopant in the base material by the ALDmethod comprises: performing a first cycle to introduce a nitrogenprecursor and a first metal precursor to form a metal nitride layer; andperforming a second cycle to introduce a second metal precursor on asurface of the metal nitride layer.
 18. The method of claim 17, whereinthe nitrogen precursor comprises NH₃, N₂, N₂O, or a combination thereof,the first metal precursor comprises TiCl₄, TaCl₅, or a combinationthereof, the second metal precursor comprises trimethylaluminium (TMA),aluminium chloride (AlCl₃), triisobutylaluminum (TIBA,Al(CH₂CH(CH₃)₂)₃), WF₆, TaCl₅, or a combination thereof.
 19. The methodof claim 14, wherein doping the one of the first, second, and third workfunction layers with the dopant in the base material by the ALD methodcomprises: performing a first cycle to introduce a first nitrogenprecursor and a first metal precursor to form a metal nitride layer; andperforming a third cycle to introduce a second nitrogen precursor and asecond metal precursor on a surface of the metal nitride layer.
 20. Themethod of claim 19, wherein each of the first and second nitrogenprecursors comprises NH₃, N₂, N₂O, or a combination thereof, the firstmetal precursor comprises TiCl₄, TaCl₅, or a combination thereof, thesecond metal precursor comprises trimethylaluminium (TMA), aluminiumchloride (AlCl₃), triisobutylaluminum (TIBA, Al(CH₂CH(CH₃)₂)₃), WF₆,TaCl₅, or a combination thereof.